Foam Insulation

Earlier I dealt with various forms of fibrous insulation, which have important characteristics in common, including the fact that their insulating value depends on the amount of still air their entrap. Foams are different: most entrap some other gas. They come in two forms: spray foams and rigid foam board. All have very good R values.

Spray foam have R-values up to 6.5 per inch and, unlike fibrous insulation, they are very good at air sealing as well. Most insulating foams are urethane-based. Urethanes are a large group of organic compounds derived from carbamic acid (NH2COOH). Carbamate esters (e.g., ethyl carbamate) are also called urethanes. Most urethanes come from petroleum stocks, however there is also a new type of urethane made from soybean shells.

Spray foams are available in low density open cell and high density closed cell varieties. Both varieties are excellent insulators and air barriers, however low density foams are somewhat more permeable by water vapor. The foams are formed as their chemical ingredients are “fluffed up” by a blowing agent introduced as they are applied. Older blowing agents were ozone-destroying compounds. Those have been gradually replaced. There is even a foam whose blowing agent is water. During application, the volume of open cell foams increases up to 100 fold, while closed cell varieties expand 30 to 35 fold. As a foam expands, it fills even the tiniest of cavities, creating a continuous airtight seal. For this reason spray foam is by far the best product to stop air leakage in a building. Cementitious spray foams are the newest entrants on the market. One of these is made of air water and cement and has an R value of about 3.9 – not too much better than fibrous insulation. As applied, this foam has the consistency of whipped cream which hardens as it dries.

Rigid foam insulation is another alternative. Extruded polystyrene (XPS) has been used to insulate foundations for many years. Rigid foam-board insulation applied to the exterior of a frame wall can improve thermal performance significantly, not only by increasing the R-value of the wall but also eliminating thermal bridging through the framing members. In addition to XPS, a close-cell polystyrene suitable for moist applications, there are two other varieties of foam board: expanded polystyrene (EPS) and polyisocyanurate.

EPS is made from rice-like pellets expanded inside the walls of a giant “press” by the addition of steam or pentane into a sheet product. XPS used to be made using a chlorofluorocarbon (CFC), any of several volatile organic compounds composed of carbon, fluorine, and chlorine, in which the chlorine is destructive of stratospheric ozone, our main protective layer from ultraviolet ration, a cause of skin cancer. When CFCs also contain hydrogen in place of one or more chlorines, they are called hydrochlorofluorocarbons, or HCFCs. CFCs were originally developed as refrigerants during the 1930s. Some of these compounds, especially trichlorofluoromethane (CFC-11) and dichlorodifluoromethane (CFC-12), found use as aerosol-spray propellants, solvents, and foam-blowing agents. They are well suited for these applications because they are nontoxic and nonflammable and can be readily converted from a liquid. HCFCs are 90% less environmentally damaging than CFCs, but they still contain chlorine and as such threaten the ozone layer.

Polyisocyanurate boards, the highest R-value foam (R-7.5) available, have the same problem. These boards usually come covered by an aluminum foil skin that reduces the off-gassing to a minimum. Over time, however, these boards still lose some insulating value: aged polyisocyanurate boards usually have an R-value of 6 to 6.5 per inch, still about double of the R-value of fibrous insulation.

Foam insulation presents us with a classic choice between “lesser evils”. Most foams contain ingredients derived from petroleum. On the other hand, they deliver almost double the R value of other insulation, reducing harmful emissions generated by heating a building. On balance using foam insulation is an environmental winner, especially since there is no technical reason why, in future, these foams could not be made from plant-based ingredients.

Finally, there is cost. Foams are among the most expensive option for residential insulation. On the other hand, over the long life-cycle of a building, they are hands-down economic winners since they can drastically cut, and even eliminate our heating fuel bills.